Project Summary/Abstract During development, the actomyosin cytoskeleton serves as a machine to generate contractile force. Sculpting tissues requires this force-generating machine to be precisely regulated in both space and time. The small GTPase RhoA coordinates both myosin motor activation and actin filament assembly, to promote contractility. RhoA is pivotal in regulating actomyosin contractility in numerous developmental contexts, including epithelium folding. One common feature of actomyosin contractility, during both Drosophila gastrulation and Xenopus neural tube closure, is that actomyosin exhibits oscillatory dynamics. Actomyosin dynamics are critical for epithelial folding in both Drosophila gastrulation and the Xenopus neural tube. In Drosophila, these dynamics require negative regulation of RhoA via a GTPase Activating Protein (GAP). Aim 1 of this proposal aims to understand how this Drosophila GAP is regulated to promote actomyosin dynamics and tissue folding. In addition, we have discovered a novel oscillatory behavior for an upstream guanine nucleotide exchange factor (GEF), which activates RhoA. Aim 2 of this proposal seeks to understand the mechanism that regulates this Drosophila GEF and, thus, paces actomyosin contractility. Overall, these two aims will determine how the partnership of a specific GEF and GAP precisely regulate RhoA signaling to promote tissue folding. Because Rho-family GTPases are critical regulators of tissue structure and there are many GEFs and GAPs for Rho-family GTPases, Aim 3 will determine the regulatory logic that influences GTPase activity for other morphogenetic processes in Drosophila. First, we will examine a change in tissue integrity, epithelial-to-mesenchymal transition (EMT). We will determine mechanisms by which GTPase regulation promotes this transition and, thus, loss of epithelial integrity. Finally, we will perform a screen to identify additional combinations of GEFs and GAPs that regulate other tissue shape changes. Our overall goal is to better define how Rho-family GEFs, GAPs, and their cognate GTPases function to precisely control tissue shape and integrity.